As part of a program to define the phenotypic properties of malignant and/or differentiating cells, we have explored a variety of cell surface probes which might be useful in identifying, characterizing and isolating cell populations which may be unique or of special interest. This proposal concerns merocyanine 540 (MC), a fluorescent dye which stains selected cell mebranes. We have shown, using fluorescence microscopy and flow microfluorimetry (FMF), that MC stains the circulatory leukocytes from leukemic patients but not the corresponding cells from normal individuals, and interacts with normal hemopoietic progenitor cells. The selective staining of leukemic cells has been employed in relevant clinical settings in which we have demonstrated that the staining reaction is a useful adjunct to existing methods for monitoring the course of leukemia and is of significant prognostic value in the prediction of the tendency toward relapse or remission. The goals of the current proposal are: (1) to continue and expand our evaluation of the clinical applications of the staining reaction in leukemia and other neoplastic disorders; (2) to characterize the dynamic changes in MC staining patterns during the clinical course of leukemia; and (3) to explore possible applications of the MC staining reaction to the isolation and characterization of hemopoietic progenitor cells.